In order to provide multi-channel voice and data communications over a broad geographical area, wireless (e.g. cellular) communication service providers currently install transceiver base-stations in protected and maintainable facilities (e.g. buildings). Because of the substantial amount of hardware currently employed to implement the signal processing equipment for a single cellular channel, each base-station is typically configured to provide multichannel communication capability for only a limited portion of the frequency spectrum that is available to the service provider. A typical base-station may contain three to five racks of equipment which house multiple sets of discrete receiver and transmitter signal processing components in order to service a prescribed portion (e.g. 48) of the total number (e.g. 400-30 kHz) channels within an available (e.g. 12 MHz) bandwidth. The receiver section of a typical one of a base-station's plurality (e.g. 48) of narrowband (30 kHz) channel units is diagrammatically illustrated in FIG. 1 as comprising a dedicated set of signal processing components, including a front end, or down-conversion section 10, an intermediate frequency (IF) section 20 and a baseband section 30.
The front end section 10 is comprised of a low noise amplifier 11 to which an antenna 12 at the transceiver site is coupled, a radio frequency-to-intermediate frequency (RF-IF) down-converting mixer 13 and an associated IF local oscillator 15. The IF section 20 is comprised of a bandpass filter 21 to which the output of mixer 13 is coupled, an amplifier 23, an if-baseband mixer 25 and an associated baseband local oscillator 27. The bandpass filter 21 may have a bandwidth of 100 kHz centered at a respective one of the 400-30 kHz sub-portions of a 12 MHz wide cellular voice/data communication band, diagrammatically illustrated in the multichannel spectral distribution plot of FIG. 2.
The baseband section 30 contains a lowpass (anti-aliasing) filter 31, an analog-to-digital (A-D) converter 33, a digital signal processing unit 35 which functions as a demodulator and error corrector, and an associated telephony (e.g T1 carrier) unit 37 through which the processed channel signals are coupled to attendant telephony system equipment. The sampling rate of the A-D converter 33 is typically on the order of 75 kilosamples/sec. The narrowband channel signal as digitized by A-D converter 33 is demodulated by digital signal processing (DSP) unit 35 to recover the embedded voice/data signal for application to telephony carrier unit 37. (A similar dedicated signal processing transmitter section, complementary to the receiver section, is coupled to receive a digital feed from the telephony system equipment and output an up-converted RF signal to the transceiver site's antenna.)
For a typical urban service area, in order to optimize service coverage within the entire bandwidth (e.g. 12 MHz) available to the service provider, and to ensure non-interfering coverage among dispersed transceiver sites at which the base-stations are located, cellular transceiver sites are customarily geographically distributed in mutually contiguous hexagonal cells (arranged in a seven cell set). Thus, each cell has its own limited capacity multi-rack base-station that serves a respectively different subset of the available 400 channels, whereby, over a broad geographical area, the frequency allocation within respective cells and the separation between adjacent cell sets may be prescribed to effectively prevent mutual interference among any of the channels of the network.
It will be readily appreciated that, since every channel has components spread over multiple equipment racks, such as those that make up a typical channel receiver section described above with reference to FIG. 1, and thus the cost and labor in geographically situating, installing and maintaining such equipment are not insubstantial. Indeed, the service provider would prefer to employ equipment that would be more flexible both in terms of where it can be located and the extent of available bandwidth coverage that a respective transceiver site can provide. This is particularly true in non-urban areas, where desired cellular coverage may be concentrated along a highway, for which the limited capacity of a conventional 48 channel transceiver site would be inadequate, and where a relatively large, secure and protective structure for the multiple racks of equipment required is not necessarily readily available.
Although wideband receivers have been used in certain other applications in the past, there are perhaps several reasons why they have not found widespread use in multichannel systems such as cellular and other PCS systems. One such concern has to do with the fact that each channel signal being received is a digitally encoded signal consisting of a series of symbols. In such an instance, the recovery algorithms used by the digital signal processor 35 typically require that the samples provided by the A-D converter 33 be taken at or near the time of peak amplitude of each symbol, in order to maximize the probability of correctly detecting each symbol.
In the prior art systems as discussed above, synchronization of a local clock to the optimum sampling time is fairly straightforward. That is because each channel is processed separately, and thus the channel signal output by the A-D converter 33 represents information from only one channel. Thus, the local receiver clock may be synchronized by using well-known phase-locked loop (PLL) techniques to generate a clock strobe for the A-D converter 33 which is synchronized to the symbol rate.
If the system uses a wideband front end, however, such that the low pass filter 31 covers the bandwidth occupied by several channels, then signals from more than one channel will be present in the output of the A-D converter 33. In a typical cellular or other PCS system, there is no requirement that the channel signals be synchronized with each other. Thus, there is no single optimum sampling time, and conventional phase locking techniques cannot be used to synchronize the A-D converter 33.
Additionally, even if it were possible to independently control the rate at which samples of the multiple digital channel signals are taken, this rate typically cannot be guaranteed to be an integer multiple of the symbol rate. This exacerbates the problem of properly synchronizing the sampling of the symbols in each digital channel signal, since even a very small difference in the rate of the digital symbols will quickly accumulate over the duration of even a short-duration channel signal, thereby again skewing the sample timing.